Why? The small business remains liable for any unpaid employment taxes.

Many small business employers hire third-party payroll service providers to perform their payroll processing functions and tax-related duties. This may include the service making employment tax deposits through the Electronic Federal Tax Payment System (EFTPS), a free way to pay federal taxes through a secure government website.

Why the IRS says you should monitor tax payments made by a tax management service provider

According to the IRS, the most important reason a small business should monitor its tax deposits is this:

In most cases, the small business remains liable for any unpaid employment taxes. This includes any penalties and interest resulting from underpayment, even if you use a third-party payroll service provider.